Abstract: Trivalent neodymium (Nd³⁺) is the most successful type of active ion for solid-state lasers and thus far has been made to lase in more types of crystal and glass hosts than any other ion. It can operate as either a pulsed or continuous-wave laser with a sharp emission line. The most common emission wavelength is near 1μm, but there are several possible laser transitions in the near-infrared spectral region, and in addition a near-ultraviolet laser line. Although the effects of different host environments on the spectroscopic properties of Nd³⁺ are more subtle than those for transition-metal ions, they can cause significant differences in lasing characteristics through changes in physical processes such as radiative transition strength, radiationless decay probabilities, excited-state absorption, and cross relaxation quenching. The Nd ion when doped into a solid-state host crystal produces the strongest emission at a wavelength just beyond 1μm. The two host materials most commonly used for this laser ion are yttrium aluminium garnet (YAG) and glass. At room temperature the 1.064μm radiative transition is homogeneous broadened with a narrow emission line width of 0.45nm and the upper level lifetime is 230μs. Nd can be doped to very high concentration in glass. The outstanding practical advantage compared to crystalline materials is the tremendous size capability for high-energy applications. The fluorescent lifetime is approximately 300μs, and, the emission line width is 18-28nm.

Abstract: A method and experimental setup intended for measurement the amplitude and phase of acoustic field in the near zone a scatterer are described. The results of measurement the scattering characteristics of low-frequency sound signals scattered by elastic cylindrical shells are analyzed.

Abstract: The propagation of light on a set of null paths in a pseudo-Riemannian manifold is studied using a variational principle of extremal energy. This method is compared with those based on the geodesics and Fermat principles. For the static space-time all three variational methods give the same solutions. The Fermat principle for stationary gravitational fields is identical to the principle of variation of energy of light-like particles. The approach proposed here is applied to a flat expanding space, corresponding to the LCDM-model, and generalized Gödel space-time. In the first case a solution obtained by the method of energy variation coincides with the geodesic method, while in the second case they give different solutions.